The extent of compressed air generation in industry, particularly in factory-type installations, is very large and such compressed air generation uses a large amount of input energy and a large amount of water for cooling. For example, in the United States alone the annual power requirement for compressed air generation is estimated to be 13.8 billion horsepower hours, which is equivalent to about one billion gallons of fuel oil. And, an immense amount of cooling water is used (up to 200 gallons per day per horsepower), which represents a disposal problem which is costly both in sewer charges and heat energy wasted.
A large rise in internal energy during the air compression process is, of course, manifested in the high temperature of the outlet stream (both air and the oil such as in oil-flooded screw compressors) from the typical compressor. While the high temperature associated with adiabatic compression is totally undesirable, it is nevertheless an inescapable, inherent feature of the compression process; the increase in internal energy corresponds simply to the mechanical work input of the process.
The presence of waste heat in air compressor systems is not an academic one, since the removal of heat from the compressed air stream is necessary to lower the temperature and cause moisture to condense from the stream prior to ultimate use of the compressed air, and to cool the separated recirculating oil stream associated with oil-flooded compressor designs. High pressure air typically must be dry for the usual applications; therefore, cooling of the heated air stream coming from the compressor is required.
Water cooling of the hot compressed air stream and recirculating oil stream is the traditional, though not exclusive, practice. The use of ambient air for after-cooling of the compressed air stream and the recirculating oil stream is an attractive alternative to water-cooling, and it is in practice in certain installations where it is feasible. Generally, air-cooling equipment is more expensive than is water-cooling equipment, but the additional expense for equipment may be offest by not having to purchase water and not having to pay for its disposal.
A more serious drawback to air-cooling, however, is that ambient air is often dirty and leads to unacceptable fouling of heat exchange surfaces. This seems to be the principal factor which leads to the selection of water-cooled equipment for many applications. The large required quantities of dust-laden ambient air cannot be suitably and economically cleaned, so air-cooling is principally used only in those applications where relatively clean ambient air is available.
Besides the need for clean ambient air for air-cooling, it should be stressed that the quality of supply air (to be compressed) to compressors is also important. The need for high standards of compressed air purity for many uses is obvious, particularly for food processing, pharmaceutical manufacturing, and other highly controlled operations. In many such applications dirty compressed air is totally unacceptable.
Dirty intake air can also create undue maintenance expense. Some compressors, such as centrifugal and rotary vane compressors, have a low tolerance for particulate-laden air, and will malfunction early when exposed to it.
Another problem of the prior art relates to energy waste and efficiency of energy usage. Industries which require compressed air, like most other industries, are facing rapidly escalating energy costs. The typical water-cooled compressed air systems throw away much of the heat energy in the cooling water as it is drained into a sewer system or elsewhere. The alternative of air-cooling offers not only the benefit of eliminating water and disposal costs, but offers the possibility of using heated cooling air for direct heating of factory space. The recovery and use of waste heat, normally discarded with traditional water-cooling, is technically feasible and has been recognized in the art. The disclosed invention makes possible such recovery and use of waste heat even in relatively dirty environments where such has not previously been feasible.
Another problem in certain compressed air systems of the prior art has been the high noise levels often associated with compressor equipment. High noise levels are bothersome and dangerous to people working in the area and can even be harmful to property. One expensive solution to the problem has been to place compressor equipment in rooms separate from the work areas. This, however, is costly and reduces flexibility in factory space allocation.
Another more limited solution to the noise problem has been the use of compressor cabinets. Cabinets, however, have tended to limit accessibility to working components, and metal cabinets have been susceptible to corrosion. A need has existed for improved noise attenuation in compressed air systems.